For the numerical simulation of multi-physics problems in geo- and hydro science, simulation tools are required which allow for (hydro-thermal-mechanics, fluid-structure interaction, etc.) which allow for the coupling of different physical quantities (fluid velocity, temperature, displacements, stresses, etc.) along interfaces or within volumes. In this project, we develop transfer operators which are based on a variational approach, i.e. a diskrete L2-projection. In contrast to standard approaches, where interpolation is used, the variational transfer in general is more stable and therefore preferable. The goal of this sub-project is the development of a coupling library, which then can be used within the different software frameworks within the SCCER-SoE.